This application is based on an application No. 2000-69846 filed in Japan, the content of which is hereby incorporated by reference.
(1) Field of the Invention
The present invention relates to an image forming apparatus, such as a copier and a printer, and particularly relates to a transfer technique used by an image forming apparatus in which an image is formed by transferring images formed using a plurality of image forming devices onto a transfer medium at different positions.
(2) Description of the Related Art
In these days, so-called tandem-type copiers are becoming increasingly common, which comprises a plurality of image forming devices image-forming toner images with different reproduced colors of cyan(C), magenta(M), yellow(Y), and black(K) are provided in a line along the travelling direction of the transfer belt.
As an example of such a tandem-type copier, Japanese Laid-Open Patent Application No. 6-110343 teaches the following apparatus (the first prior art).
FIG. 1 is a schematic diagram showing a construction of an image forming section of a tandem-type copier in the first prior art. As shown in FIG. 1, this tandem-type copier comprises a plurality of image forming devices 110C to 110K forming toner images with colors of C, M, Y, and K and provided in a line along the travelling direction of a transport belt 121, and a plurality of transfer rollers 125C to 125K for serving as transfer members provided in the positions opposite to photosensitive drums 111C to 111K of image forming devices 110C to 110K via the transport belt 121, respectively. Constant voltage power supplies 170C to 170K are connected to the transfer rollers 125C to 125K, respectively. As a recording sheet S is transported by the transport belt 121, images with differently developed colors are superimposed to form a color image by applying transfer voltages to the transfer rollers 125C to 125K sequentially.
Here, the transfer voltage applied to each of the transfer rollers 125C to 125K is set so that a voltage across the transfer roller at the lower stream is higher than the upper stream. That is, the transfer voltage is increased gradually higher according to the arranged order of the transfer rollers. The transport belt and the recording sheet become charged in the former transfer step and then transported to the next transfer position with being charged. In addition, the thickness of toner on the recording sheet is increased as the transfer step goes to the lower stream. Consequently, unless a higher voltage is applied to the transfer roller in the lower stream, the same transfer efficiency cannot be maintained on the recording sheet.
Therefore, in the first prior art, the transfer rollers 125C to 125K have constant voltage power supplies 170C to 170K respectively, where voltages are set so as to increase gradually as the transfer step goes to the lower stream, whereby the stability of transfer efficiency of the toner image with each color can be obtained.
In the above first prior art, however, a plurality of constant voltage power supplies are required as many as the number of the transfer rollers, so that an increase in the cost of the tandem-type copier and upsizing thereof are inevitable.
In order to solve the above stated problem, Japanese Laid-Open Patent Application No. 9-50197 discloses another tandem-type copier (the second prior art). FIG. 2 is a schematic view showing a construction of an image forming section of a tandem-type copier in the second prior art. Each of transfer rollers 180C to 180K is connected in parallel to a constant voltage power supply 171, so that the same voltage is applied to each of the transfer rollers. The resistance value of each of the transfer rollers 180C to 180K is set so as to decrease gradually as the transfer step goes to the lower stream. Normally, a transfer roller is made up of a core metal covered with an electrically conductive rubber, whose resistance value can be varied by adjusting a quantity of carbon to be mixed therewith.
Thereby, even if the same voltage is applied to each of the transfer rollers 180C to 180K, a transfer current passing through the corresponding photosensitive drum among 111C to 111K can be increased gradually as the transfer step goes to the lower stream. Therefore, an effect of stabilization of the transfer efficiency can be achieved with one constant voltage power supply. In addition, a reduction of costs and downsizing for the tandem-type copier can be realized.
In the above second prior art, however, four kinds of transfer rollers 180C to 180K whose resistance value is different from each other have to be prepared. Consequently, there is a problem that a manufacturing cost is increased as compared with the copier using one type of transfer rollers.
Moreover, it is very difficult, in fact, to adjust the subtle resistance value of each electrically conductive roller, so that the manufacturing process has to be controlled precisely. In addition, granted that proper resistance values can be obtained, there is a problem that the proper ratio of each resistance value of the transfer rollers cannot be maintained for a long time, because the degree of the change in the resistance values may vary with each other due to the difference in the contents of carbon.
The problems stated above may occur in a like manner when electrically conductive transfer blades or the like are adopted as transfer members instead of the transfer rollers.
The object of the present invention is to provide an image forming apparatus wherein an appropriate voltage can be applied to each transfer member while low-cost and downsizing of the apparatus can be realized with a compact structure.
The above object of the present invention can be achieved by an image forming apparatus made up of: a plurality of image forming devices; a plurality of transfer members which are provided in one-to-one correspondence with the image forming devices, and sequentially transfer images formed by the image forming devices onto a transfer medium; a transfer power supply device which generates a predetermined voltage for a transfer operation; and a voltage distributing circuit which is arranged between the transfer power supply device and each of the transfer members; and distributes different voltages to one-to-one correspondence with the transfer members.
The above object of the present invention can be also achieved by an image forming apparatus made up of: at least three image forming devices; a plurality of transfer members which are provided in one-to-one correspondence with the image forming devices, and sequentially transfer images formed by the image forming devices onto a transfer medium; a plurality of transfer power supply devices which generates predetermined voltages for transfer operations, a number of the transfer power supply devices is one less than a number of the transfer members; and a voltage distributing circuit which is arranged between two or more transfer members and a first transfer power supply device among the transfer power supply devices and distributes different voltages to one-to-one correspondence with two or more transfer members.